1. Field of the Invention
The invention relates to a lens position control method, a lens position control apparatus, a cutting method, and a cutting apparatus which are applied to position control of an objective lens in a cutting step upon manufacturing of, for example, a high-density optical disc.
2. Description of the Related Arts
As a high-density optical disc, for example, there has been proposed an optical disc having a recording capacity of about 25 Gbytes for a single layer of one side or having a recording capacity of about 50 Gbytes for double layers of one side. In such an optical disc, in order to decrease a spot diameter of a beam for recording and reproduction, a wavelength of a light source is set to 405 nm and a numerical aperture NA of an objective lens is set to a large value of 0.85. In the high-density optical disc, a beam spot area can be reduced to about ⅕ of that of a DVD. Further, since an angular error (called a tilt margin) which is permitted for an inclination from 90° of an angle formed between the disc surface and an optical axis of a laser beam decreases as a result of an increase in the numerical aperture NA of the objective lens, a cover layer covering an information layer is thinned to 0.1 mm. In the case of a read only disc, the information layer is a reflecting layer or a translucent reflecting layer on which pits have been formed. In the case of a recordable disc, the information layer is a recordable layer such as a phase change layer or the like on which grooves have been formed.
FIGS. 1A and 1B show structures of examples of a high-density optical disc to which an embodiment of the invention can be applied. FIG. 1A shows the structure of a single layer. Reference numeral 1 denotes a substrate made of polycarbonate (hereinafter, properly abbreviated to PC) having a thickness of 1.1 mm.
Pits of a master disc have been transferred onto the surface of the substrate 1 by injection molding. The substrate 1 is coated with a reflecting film 2. A cover layer 3 as a light transmitting layer having a thickness of 0.1 mm has been adhered onto the reflecting film 2. The cover layer 3 is formed by a method whereby a PC sheet 5 which has previously been punched is adhered with a UV (ultraviolet rays) hardening type adhesive agent 4 and a surface portion of the PC sheet 5 is coated with a hard coating 6.
FIG. 1B shows the structure of double layers. In a manner similar to the single-layer structure, FIG. 1B shows the disc having two information layers each having such a structure that the reflecting film 2 as a total reflecting film is formed on a substrate of 1.1 mm, a translucent reflecting film 8 is formed on a light transmitting layer 7 called an intermediate layer formed on the reflecting film 2, and further the cover layer 3 is adhered onto the translucent reflecting film 8. The reflecting film 2 is formed in a depth of 100 μm when seen from the incident direction (on the side of the hard coating 6) of the laser beam and the translucent reflecting film 8 is formed in a depth of 75 μm.
In the case of the one-side double-layer disc shown in FIG. 1B, the reflecting film 2 existing in the depth of 100 μm when seen from the incident direction of the laser beam is defined as a reference layer (the 0th recording layer; called an L0 layer) and the recording layer added in the depth of 75 μm is defined as a first recording layer (called an L1 layer).
An outline of a manufacturing method of the high-density optical disc will be described with reference to FIG. 2. Reference numeral S1 denotes a molding step by a stamper for L0. In a mastering step, a resist on the substrate is exposed by the laser beam, a disc-shaped master disc having concave/convex portions corresponding to pits or grooves on the resist is formed by development, and a stamper for the L0 layer made of a metal is formed from the disc-shaped master disc. In the molding step S1, the substrate 1 is formed by injection molding by using the formed stamper and a molding material such as PC. The molded substrate 1 is cooled by a cooling apparatus.
Subsequently, a film forming step S2 of the reflecting film 2 as a total reflecting film of the L0 layer is executed. In the film forming step S2, a component of a target is deposited onto the substrate by using a sputtering apparatus. Subsequently, an intermediate layer forming step S3 is executed. The light transmitting layer 7 as an intermediate layer is formed by adhering a sheet or by a spin coating method.
Subsequently, an L1 pattern transfer step S4 is executed. In the L1 pattern transfer step S4, the pits or grooves of the L1 layer are transferred onto the UV hardening type sheet by using an L1 stamper manufactured by another step by, for example, a pressure transfer. In the case where the intermediate layer is formed by the spin coating method, the pits or grooves of the L1 layer are transferred to the UV hardening resin in a semi-hardening state.
In a UV hardening step S5 of the first time, UV (ultraviolet rays) are irradiated to the UV hardening type sheet or the UV hardening type resin by using a UV irradiating apparatus and a pattern of the transferred pits is fixed. In a disc peeling step S6, the disc is peeled off from the stamper by using a disc peeling apparatus. The “disc” mentioned here denotes the disc in which the pits or grooves of the L1 layer have been transferred to the light transmitting layer 7.
In a film forming step S7 of the L1 layer, the L1 layer, for example, the translucent reflecting film 8 is formed onto the formed pattern of L1. In the film forming step S7 of the L1 layer, a component of pure silver or silver alloy as a target is deposited onto the substrate by using the sputtering apparatus by a DC sputtering method in a manner similar to the foregoing L0 layer.
Subsequently, a cover layer forming step S8 is executed. The cover layer forming step S8 includes three steps of a UV hardening type adhesive agent coating step, a PC sheet adhering step, and a UV hardening step of the second time. In the UV hardening type adhesive agent coating step, the surface of the translucent reflecting film 8 is coated with a UV hardening type adhesive agent. The PC sheet is adhered to the disc. In the UV hardening step of the second time, the UV hardening type adhesive agent is adhered and the cover layer 3 is formed. In a forming step S9 of a hard coating layer, the hard coating layer is formed.
The presence or absence of defects of the disc with the double-layer structure formed by the manufacturing steps as mentioned above is inspected by using an inspecting apparatus, so that the disc is completed. A disc with the single-layer structure is also manufactured by the steps excluding the steps S3 to S7 regarding the L1 layer.
A technique which can solve the problem occurring in the case of using the organic resist in the related art and manufacture the high-density optical disc has been disclosed in Patent Document 1 (JP-A-2003-315988). There has been shown a technique that, according to an inorganic resist material made of incomplete oxide of a transition metal disclosed in Patent Document 1, a pattern smaller than the spot diameter can be exposed even by a visible laser of about 405 nm owing to heat recording characteristics. An attention is paid to such a technique as a technique which is useful for a mastering technique of the optical disc corresponding to the realization of the high recording density.
The incomplete oxide of the transition metal used here denotes a compound whose oxygen content is deviated in such a direction that it is smaller than a stoichiometric composition according to a valence number which the transition metal can have, that is, a compound in which an oxygen content in the incomplete oxide of the transition metal is smaller than that of the stoichiometric composition according to the valence number which the transition metal can have. In the incomplete oxide of the transition metal, since a latent image forming portion by the exposure has been oxidation-altered, it is soluble into an alkali developer and microfabrication of the master disc for the optical disc can be realized.
An embodiment of the invention relates to a positioning method of an objective lens in a cutting apparatus in the case of using such an inorganic resist. In the cutting apparatus, since a spiral track is formed by feeding precision of a master disc in which the inorganic resist has been formed as a film onto a substrate such as a silicon wafer or the like, tracking control is not made but only control in the focusing direction (focusing servo) is made. The focusing control is made by a method similar to the method such as an astigmatism method or the like which is used in a reproducing apparatus.
Since a lead-in range of the focusing control is limited, first, a distance between the objective lens and the surface of the master disc is brought in a range where the focusing servo can be pulled in. Control for such a purpose is called positioning control and is made by allowing the position of the master disc to approach the objective lens. The focusing servo is made operative after completion of the positioning. In the focusing servo, the vertical position of the objective lens is feedback-controlled so that an in-focus state can be obtained.
In the cutting apparatus disclosed in Patent Document 1, since the commercially available objective lens of the small diameter is used, generally, there is a tendency that a working distance of the objective lens decreases. The working distance is a physical vertex portion of the objective lens that is nearest to the focal position. For example, when the working distance is equal to 150 μm, if the positioning of the objective lens is not performed at high precision in the initial adjustment after the master disc was set, there is a fear that the objective lens collides with the master disc or an inconvenience occurs in the focusing servo. It is, therefore, important to set the objective lens at the time of the initial adjustment so that the focal position of the objective lens coincides with the recording surface of the master disc.
The positioning method in the related art will now be described. A focal depth of the objective lens can be calculated by a value λ/(2NA)2 obtained by dividing a wavelength λ of light by the square of the numerical aperture NA of the lens. In the cutting apparatus in the related art, in order to converge the spot of the laser beam for exposure, the wavelength is shortened and the numerical aperture is increased. Therefore, the focal depth becomes very small.
According to the method disclosed in Patent Document 1, the lens of the large numerical aperture and the light source of the short wavelength are not necessary in the related art. For example, when λ=400 nm and NA=0.85, the focal depth of 0.14 μm can be obtained. The focal depth denotes a range where the focal point is satisfactory even if the objective lens moves on an optical axis. Generally, photodetecting sensitivity in a photodetector of an optical pickup is set so that up to a value which is several times as large as the focal depth can be detected. The range where the focal depth can be detected is called a detectable range. For example, the detectable range is assumed to be 2.5 μm.
In the case of making positioning control of the objective lens by moving either the objective lens or the master disc, only when the distance between them lies within the foregoing detectable range, the detection output can be derived from the photodetector. When the distance is out of the detectable range, since the light reception amount decreases, the detection output is not obtained and it is difficult to sense the existence of the master disc. Therefore, when the detection output is obtained in the detectable range, by stopping the relative movement of the objective lens or the master disc, the focal distance of the objective lens almost coincides with the position of the surface of the master disc. After that, the focusing servo is made operative and the exposing operation is executed.
However, if the operation to stop the movement by using the detection output in the detectable range fails, the objective lens and the master disc collide. As a countermeasure for such a trouble, it is necessary that a safety stopping apparatus for stopping the movement at a distance where both of the objective lens and the master disc do not collide is provided so that the objective lens and the master disc do not collide. In the cutting apparatus in the related art, the following methods have been used.
As a first method, as shown in FIG. 3, there is a method of positioning the objective lens on the basis of an output signal of a distance sensor such as an optical distance sensor 23 which is moved integratedly with an objective lens 22. It is a method whereby the distance sensor 23 detects a distance from a master disc 21 and, when the detected distance reaches a set value, the movement is stopped.
The second method is a method of stopping the movement at a predetermined position by using a distance detecting function of a Z-axial stage for supporting the master disc 21.